Geotech 95141.00.R.001

                                                                                                          Page 8 of 10




The upper 1 m should be ignored in pile capacity estimates to allow for load development and
disturbance of soils.

Experience indicates that properly designed and constructed piles sized using the above parameters
appropriately factored are likely to undergo settlements in the order of 1% of the pile diameter.



6.5    Retaining Walls

The design of flexible and rigid retaining walls could be undertaken using a triangular pressure
distribution and the earth pressure parameters given in Table 3. Flexible walls are those which are free
to rotate or tilt (such as cantilevered walls) and should be designed using active (Ka) earth pressure
coefficient. Rigid walls are those which are restrained against rotation or tilt (ie. single anchored/propped
walls) and should be designed using the at-rest earth pressure (Ko).

Passive resistance (Kp) at the toe of the wall should be ignored in the zone where future disturbance
(eg. services trenches) could occur.

Table 3: Earth Pressure Coefficients (non-sloping crest backfill)
                Material                        Unit        Friction          Active      At Rest        Passive
                                              Weight         Angle             Ka           Ko             Kp
                                              (kN/m3)      (degrees)
      Very loose to loose sand                   17             28                 0.35    0.50             NR
 Controlled granular fill*, medium
                                                 18             33                 0.30    0.45             3.3
           dense sand
Notes: * Assumes fill material compacted under ‘Level 1’ inspection and testing.
       NR = Not Recommended


Allowance should be made for hydrostatic pressure build-up behind the retaining wall. It is
recommended that all retaining walls be drained for full height in order to minimise hydrostatic pressure
build-up behind the wall. Additional guidelines on wall drainage are provided in Appendix G of
AS 4678 (2002).

Also, allowance for surcharge loads and sloping crest should be made as appropriate. The effect of
surcharge should be included by multiplying the vertical pressure developed by the surcharge by the
appropriate lateral earth pressure coefficient as given in Table 3.

It is recommended that a factor of safety of 2 be adopted for overturning and sliding stability, and 1.5 for
global stability in the design of all retaining walls.

For limit state design methods, the ultimate parameters provided in Table 3 will need to be factored in
accordance with AS 4678. Guidance on the selection of material strength partial factors is provided in
Section 5.2 of AS 4678 and is dependent upon the nature and state of the insitu soil or fill material.

Preference should be given to adopting thin soil layers and using small hand-controlled compaction
equipment during backfilling against retaining walls. This is in order to limit the stress applied to the walls




 Geotechnical Investigation, Proposed Youngcare Facility                                            95141.00.R.001.docx
 32 Viewland Drive, Noosa Heads                                                                            January 2021